1. Field of the Invention
The present invention relates to a wavelength multiplexing/demultiplexing apparatus that is used in wavelength division multiplexing (WDM) communications for multiplexing a plurality of light with different wavelengths to demultiplex, for each wavelength, the multiplexed light that has been transmitted through a single optical fiber, or for multiplexing a plurality of light with different wavelengths to input the multiplexed light to a single optical fiber, and in particular, relates to a technique for reducing a loss in the wavelength multiplexing/demultiplexing apparatus using an optical waveguide of a substrate shape that is formed by enclosing, by a cladding, a core having a refractive index higher than that of the cladding.
2. Description of the Related Art
A configuration example of a wavelength multiplexing/demultiplexing apparatus using a conventional optical waveguide is shown in FIG. 10, in which (a) is a plan view, and (b), (c) and (d) are cross sectional views taken along lines Axe2x80x94A, Bxe2x80x94B and Dxe2x80x94D in the plan view, respectively.
As illustrated in (b) of FIG. 10, the optical waveguide in the conventional wavelength multiplexing/demultiplexing apparatus comprises a cladding 102 and a core 103 respectively formed on a substrate 101. The core 103 has a refractive index higher than that of the cladding 102 and is enclosed by the cladding 102.
Patterns 2-6 shown in (a) of FIG. 10 are formed by core patterns, wherein 2 is an input waveguide, 3 is an input slab, 4 are channel waveguides, 5 is an output slab and 6 are output waveguides. Here, a connection point between the input waveguide 2 and the input slab 3 is denoted by 301, an interface between the input slab 3 and the channel waveguides 4 is denoted by 302, an interface between the channel waveguides 4 and the output slab 5 is denoted by 502, and an interface between the output slab 5 and the output waveguides 6 is denoted by 504. Further, in (b) of FIG. 10, thickness and width of the core 103 constituting the input waveguide 2 are denoted by t and w2, respectively. Still further, in (d) of FIG. 10, spacing between each core 103 constituting each of the channel waveguides 4 at the interface 302 is denoted by P1, and width of each core 103 is denoted by w4. In addition, in (a) of FIG. 10, spacing between each core 103 constituting each of the channel waveguides 4 at the interface 502 is denoted by P2. Note, each core 103 has the same thickness t at all positions in the input waveguide 2, the input slab 3, the channel waveguides 4, the output slab 5 and the output waveguides 6. Further, it is assumed that the channel waveguides 4 are configured so as to be gradually longer from the lower side to the upper side in (a) of FIG. 10, and the length of each channel waveguides 4 is adjusted so that a difference between optical path from the input slab 3 and the output slab 5 in the adjacent core patterns is maintained to be fixed
The conventional wavelength multiplexing/demultiplexing apparatus is designed so that the interface 302 between the input slab 3 and the channel waveguides 4 is a circular arc interface centered at the point 301 with a curvature radius r1, the interface 502 between the channel waveguides 4 and the output slab 5 is a circular arc interface centered at a point 501 with a curvature radius r2, and the curvature radiuses of r1 and r2 of respective circular arc interfaces are equal to each other. Further, it is typical that the spacing P1 between the cores comprising the channel waveguides 4 at the input slab 3 side is equal to the spacing P2 between the cores comprising the channel waveguides 4 at the output slab 5 side.
In such a conventional wavelength multiplexing/demultiplexing apparatus, if each light with each wavelength xcex1, xcex2 and xcex3 is multiplexed to be input to the input waveguide 2, for example, the multiplexed light is demultiplexed for each wavelength xcex1, xcex2 and xcex3 to be output from each of the output waveguides 6. Conversely, if each light with each wavelength xcex1, xcex2 and xcex3 is input to each of the output waveguides 6, each light with each wavelength xcex1, xcex2 and xcex3 is multiplexed to be output from the input waveguide 2.
Incidentally, in the wavelength multiplexing/demultiplexing apparatus constituted as described above, as shown in (c) of FIG. 10, gaps G exist in a position where each of the channel waveguides 4 is connected to the input slab 3. Each of the gaps G is a factor responsible for a loss of light that is incident on the input waveguide 2 and propagated through the input slab 3 to be coupled to each of the channel waveguides 4. Therefore, it is preferable that each gap G is as narrower as possible.
On the other hand, in the channel waveguides 4, since it is necessary to define a phase difference between the light passing through each waveguide, it is required that interference between the waveguides does not occur. For this purpose, the channel waveguides 4 need to be formed so as to maintain spacing constant or above therebetween except at connection points with the input slab 3. Conventionally, by maintaining the spacing P1 constant between the channel waveguides at the connection points with the input slab 3, the spacing of the channel waveguides at other portions is also maintained constant.
As one of methods of reducing the gaps G while maintaining the spacing P1 constant or above, there is known a method of forming tapered portions 401 on the portion where the input slab 3 and the channel waveguides 4 are connected with each other, as shown in (a) of FIG. 10. In this way, in the conventional wavelength multiplexing/demultiplexing apparatus, the gaps G are made narrower to reduce a connection loss between the input slab 3 and the channel waveguides 4. Note, in the configuration example in (a) of FIG. 10, tapered portions 402 are also formed on the point where the channel waveguides 4 and the output slab 5 are connected with each other.
In the conventional wavelength multiplexing/demultiplexing apparatus as described above, if a waveguide with a core having the thickness t in 7 xcexcm is used, for example, it is required that the core spacing P1 between the channel waveguides 4 at the input slab 3 side is about 18 xcexcm or more. At this time, in order to reduce the loss, the gaps G between the channel waveguides 4 may be as narrower as possible. However, due to the width of photomasking for processing and overetching in a transverse direction at the time of processing, it becomes difficult to form the gaps G into about 3 xcexcm or less according to the known technique described above. As a result, there is a problem in that, even in the case where the tapered portions 401 are formed, the loss of 1 dB or more causes at the portion where the input slab 3 and the channel waveguides 4 are connected with each other.
There are known techniques for reducing the loss in the conventional wavelength multiplexing/demultiplexing apparatus disclosed in Japanese Patent No. 2861996 and Japanese Unexamined Patent Application No. 2000-131541. Such known techniques are for reducing the loss at the connection points between the output slab and the output waveguides by performing mode matching at the connection points between the output slab and the output waveguides. Further, more typically, these techniques are for reducing connection loss at the portion where the input slab and the input waveguide are connected with each other or the portion where the output slab and the output waveguides are connected with each other. However, the known techniques described above do not refer to the mode matching at the connection points between the input slab and the channel waveguides as described above, and thus the problem about the loss reduction at such a portion has not yet been solved.
The present invention has been accomplished in view of the above problem, and it is an object of the present invention to provide a wavelength multiplexing/demultiplexing apparatus that reduces a loss at a connection portion where an input slab and channel waveguides are connected with each other, to thereby enable to realize reduction of loss in the overall apparatus.
In order to achieve the above object, according to the present invention, there is provided a wavelength multiplexing/demultiplexing apparatus comprising on a substrate: at least one input waveguide that is input with an optical signal at one end thereof; an input slab that is connected with the other end of the input waveguide at an input side interface thereof, and propagates freely the optical signal from the input waveguide in a plane direction of the substrate to an output side circular arc interface thereof; a plurality of channel waveguides that are connected to the output side circular arc interface of the input slab at one ends thereof, and have waveguide lengths different from each other; an output slab that is connected with the other end of each of the channel waveguides at an input side circular arc interface thereof and propagates freely the optical signal from each of the channel waveguides in the plane direction of the substrate to an output side interface thereof; and a plurality of output waveguides that are connected to the output side interface of the output slab at one ends thereof. This wavelength multiplexing/demultiplexing apparatus is characterized in that a curvature radius of the output side circular arc interface of the input slab is smaller than a curvature radius of the input side circular arc interface of the output slab, and further, spacing between the channel waveguides at a portion where the input slab and the channel waveguides are connected with each other is narrower than spacing between the channel waveguides at a portion where the channel waveguides and the output slab are connected with each other.
In the wavelength multiplexing/demultiplexing apparatus with such a constitution, WDM light input to the input waveguide, for example, is propagated through the input waveguide to be sent to the input slab, and is propagated freely in the input slab in a plane direction of the substrate to be extended, and then this extended WDM light is guided from the output side circular arc interface to the plurality of channel waveguides. The WDM light propagated through the channel waveguides, in which a phase difference occurs depending on a difference between the optical path lengths of the channel waveguides, reaches the input side circular arc interface of the output slab to be propagated freely in the output slab in the plane direction of the substrate. At this time, since the phase difference in the channel waveguides is different depending on the wavelengths, each optical signal of the WDM light is condensed at different points depending on the wavelengths. Then, optical signals of the respective wavelengths are propagated separately through corresponding output waveguides to be emitted from each of the output waveguides.
In the propagation process of the optical signals as described above, since the curvature radius of the circular arc interface of the input slab is set to be smaller than the curvature radius of the circular arc interface of the output slab, and further, the spacing between the channel waveguides at the portion where the input slab and the channel waveguides are connected with each other is set to be narrower than the spacing between the channel waveguides at the portion where the channel waveguides and the output slab are connected with each other, an area where each intensity distribution of light (electric field) excited by the adjacent channel waveguides, at the connection portion to the input slab, is overlapped with each other is increased, and therefore, the loss can be reduced when the light having been propagated freely in the input slab is optically coupled to each of the channel waveguides.
Further, in the wavelength multiplexing/demultiplexing apparatus described above, each channel waveguide may comprise, in the vicinity of the connection portion to the input slab, a tapered portion formed so that the width of the waveguide becomes narrower in the direction of the input slab. According to such a constitution, since the width of the intensity distribution of the excited light is further extended with respect to the width of each channel waveguide at the connection portion to the input slab, the area where each light intensity distribution of the adjacent channel waveguides is overlapped with each other is further increased so that the loss can be further reduced at the portion where the input slab and the channel waveguides are connected with each other.
Still further, each channel waveguides described above may further comprise a constant-width portion that has the constant width equal to the narrowest width of the waveguide in the tapered portion and the tapered portion may be connected to the input slab via the constant-width portion. Thus, it becomes possible to reduce the length of the tapered portion, thus enabling to miniature the size of the apparatus.
The other objects, features and advantages of the present invention will be apparent from the following description of the embodiments with reference to the accompanying drawings.